Light diffusers are extensively used to redistribute light from point or linear light sources so as to convert these concentrated light sources to more pleasing large area light sources.
In the prior art, two main approaches to accomplish this goal are practiced.
In a first approach, the light-diffusing element contains randomly distributed imperfections that refract and disperse the incoming light in all directions. Belonging to this approach are frosted elements, and white glasses and plastics. In the design of such diffusers, light-emission differences from various points of the surface decrease as the total light loss increases.
More particularly, in order to achieve high quality (homogeneous light distribution) from the emitting surface, the thickness of the diffusing elements and the concentration of dispersing particles is increased, this causes light loss within the element itself, mostly due to the increased integrated optical path of the light traversing the dispersing element and losses incurred in multiple reflections (including back reflections) from the frosted surfaces or the particles within the diffuser. Such diffusers are thus optically inefficient and their weight can become unacceptable in some applications
Representing the second approach is a refractive type light diffuser, like those often used in conjunction with fluorescent light sources, in which most of the light rays, while refracted in various directions, traverse the optical medium only once, and, as a result, only minimal optical losses occur. These diffusers usually consist of solid glass or plastic (having an index of refraction appreciably larger than 1) on which a tessellation of a geometrical three-dimensional impression is produced. The impressions are often small pyramids, other geometrical forms having faceted surfaces or lens-like protrusions, characterized by the fact that a plurality of angles of refraction are presented to light emanating from the light source.
Light wavefronts are thus refracted in various directions by the panel, resulting in diffusion of the light from a sharp source into a less concentrated form emanating from a larger area. The shortcoming of this approach is that often a strong residual image of the point (or line in the case of fluorescent lights) light source is still visible and the areal weight of the lens is appreciable. Furthermore, such diffusing panels always have an embossed outer surface where dust can accumulate causing optical absorption losses and rendering cleaning of these surfaces cumbersome.
In the prior art, Fresnel lenses are sometimes used as flat optical elements in optical systems to form real and virtual images. In these systems, the Fresnel lenses have very well-defined circular geometry and the ridges of the lenses have an exact relationship to each other (namely, they are the micro-projection on a plane of an actual convex on concave lens) so as to concentrate light, by contrast to diffusion.